Two important performance properties of gear oils are their extreme pressure performance and their air release properties. Formulated gear fluids which combine very low air release values and high extreme pressure values are highly desired.
One trend in the industry is the design of gear boxes by OEMs with smaller reservoirs and therefore smaller lubricant volumes per gear box are needed. During gear operation this puts extra stress on the gear lubricant as temperatures increase since there is less fluid available to dissipate the heat. In addition gear lubricants can entrain air and it is important that this air is released rapidly. It is common for lubricants to entrain 10% by volume of air. Entrained air can cause poor friction control, enhanced oxidation, noise, vibration and cavitation. For these reasons, fluids with lower air release values are desired. Furthermore it is important that gear lubricants have excellent extreme pressure properties to prevent welding under high contact pressures as the gear teeth mesh.
Most commercially available industrial gear lubricants are in the viscosity range ISO-150 to 1000 (i.e. 150 to 1000 cSt at 40° C.). Their air release values are >5 minutes and typically in the range 10-20 minutes at 75° C.